Here's why:
* It names a person. Nouns are words that name people, places, things, or ideas.
* It can be the subject of a sentence. For example: "The girl laughed." "Girl" is the subject performing the action.
* It can be the object of a sentence. For example: "I saw the girl." "Girl" is the object of the verb "saw."
Let me know if you have any other word questions!